The present invention relates to a process for the preparation of improved cellulose pulps giving papers with improved tensile strength, tear strength, light-scattering, and low shive content, and to an apparatus for the preparation thereof.
In the preparation of cellulose pulps, such as thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP) the fibers are laid free from each other and from lignin. The defibration process must be carried out in such a way that fiber cutting is avoided as much as possible, since long fibers give high tearing resistance in the paper that is prepared from the pulp. Fibers that still cling together form so-called shives which can cause web breaks in the paper machine or a lowering of the quality of the paper produced. In order to obtain high tensile strength, and to avoid fiber rising in offset printing when the paper is subjected to wetting by water, strong bonds between the fibers are required. To ensure fibers with good bonding ability, the fibers must be developed, i. e. treated so that the fiber wall is softened, and the surface of the fibers treated so that most of the outer thin layer, the primary wall, is removed and fibrils are loosened from the secondary wall. Thereby better contact between the secondary walls is obtained, and any residues of the lignin-rich hydrophobic middle lamella are removed. Flexible fibers are a prerequisite for achieving a paper with a smooth surface, suitable for coating, in particular for light-weight coated paper.
The pulp coming from the screening department contains both fibers that are well suited for the manufacture of paper, and some material that must either be further treated, such as incompletely treated fibers and shives, or be removed from the system, such as sand and bark particles. There is also a certain amount of fines, consisting of small pieces of the middle lamella and the primary wall, parts of fibrils from the secondary wall, parenchyme cells, and short pieces of cut fibers. Most of the fines material increases the strength and the light-scattering ability of the paper. In order to separate out fibers with good bonding ability it has been suggested to use screens or hydrocyclones. Screens separate according to particle size and hydrocyclones according to specific surface area. Screen rejects, however, also contain long fibers, which should be recovered. Rejects refining increases the bonding ability of the fibers. Factors particularly affecting the fiber fractionation capability of a hydrocyclone are pressure drop, rejects ratio, hydrocyclone geometry, and pulp slurry feed consistency.
The present invention refers to a process for the preparation of improved cellulose pulps in which defibered cellulose pulps are screened for removal of shives, fibers with low bonding ability are removed in hydrocyclones, and rejects from the hydrocyclone treatment are treated in reject refiner, which is characterized in the combination of the following characteristics:
a) the base end outflow diameter (Db) of the hydrocyclones being less than 14 mm
b) the distance (Lu) between the inner base end outflow opening and the narrowest part of the apex opening being greater than 400 mm, and
c) the ratio between the volumetric flow through the apex opening (Qa) and the volumetric flow through the inlet opening (Qf) of the hydrocyclones being controlled to lie within the interval 0.10-0.60.
According to this process it is possible to obtain satisfactory fractionation according to fiber bonding ability in hydrocyclones and prepare a pulp which yields a paper with improved tensile strength, tear strength, light-scattering, and surface smoothness.
In a modified version of the process of the invention, in which an arrangement of a centrally and axially placed blocking device (B) of circular cross section in the base end outflow opening is substituted for the parameter a) above, it is possible to further improve the process, so that it yields a paper which, in addition to improved tensile strength, tear strength, light scattering, and surface smoothness, also has a very low shive content.
This modified process thus refers to a process for the preparation of improved cellulose pulps in which defibered cellulose pulps are screened for removal of shives, fibers with low bonding ability together with remaining shives are removed in hydrocyclones, and rejects from the hydrocyclone treatment are treated in refiner, said process being characterized by the combination of the following characteristics:
a) the distance (Lu) between the inner base outflow opening and the narrowest part of the apex opening of the hydrocyclone being kept greater than 400 mm
b) the ratio between the volumetric flow (Qa) through the apex opening and the volumetric flow (Qf) through the inlet openings of the hydrocyclones being regulated to lie within the interval of from 0.08 to 0.60 and
c) the base outflow channel of the hydrocyclones being provided with a centrally and axially arranged blocking device (B) of circular cross section, the ratio of the diameter (Dd) of this blocking device to the diameter of the base outflow opening (Db) being kept within the interval of from 0.1 to 1.2.
The invention also refers to an apparatus for application of the process in which cellulose pulps are screened comprising hydrocyclones C for separation of fibers with low bonding ability and device RR for refining rejects from the hydrocyclones C, characterized by the combination of the following characteristics:
a) the base end outflow diameter Db of the hydrocyclones being less than 14 mm
b) the distance Lu between the inner base end outflow opening and the narrowest part of the apex opening of the hydrocyclones being greater than 400 mm
c) means P,V for establishing a volumetric flow Qa through the apex opening of the hydrocyclones that relates to the volumetric flow Qf through the inlet opening of the hydrocyclones such that the ratio Qa/Qf is within the interval 0.10-0.60.
The invention includes a modified apparatus for application of the process of the invention which results in a very low shive content, in which the base outflow channel of the hydrocyclones are provided with a centrally and axially arranged blocking device B of circular cross section. This modified apparatus thus refers to an apparatus for application of the process of the invention in which cellulose pulps are screened comprising hydrocyclones C for separation of fibers with low bonding ability and device RR for refining rejects from the hydrocyclones C, which apparatus is characterized by the combination of the following characteristics:
a) the distance Lu between the inner base end outflow openings and the narrowest part of the apex openings of the hydrocyclones being greater than 400 mm,
b) means P,V for establishing a volumetric flow Qa through the apex openings of the hydrocyclones that relates to the volumetric flow Qf through the inlet openings of the hydrocyclones, such that the ratio Qa/Qf is within the interval of from 0.08 to 0.60, and
c) the base end outflow channel of the hydrocyclones being provided with a centrally and axially arranged blocking device B of circular cross section, the ratio of the diameter Dd of this blocking device to the diameter Db of the base outflow opening being within the interval of from 0.1 to 1.2.
The expression xe2x80x9chydrocyclonesxe2x80x9d above and in the following is intended to mean one or several in parallel interconnected hydrocyclones including so-called multihydrocyclone aggregates.
Although especially applicable to TMP and CTMP the process and the apparatus of the invention can also be used with other types of cellulose pulps when improved bonding ability is desired, such as beaten chemical pulp and pulp made from recycled fibers.
The ratio Qa/Qf that should be within the interval 0.10-0.60, can preferably be kept within specific limits, depending of the pulp treated. For chemical pulps the ratio Qa/Qf is preferably 0.10-0.25, whereas the corresponding preferred interval for TMP is 0.20-0.40, and for CTMP 0.10-0.30.
The process of separation of fibers with low bonding ability can be carried out in one or in several hydrocyclone stages with different Qa/Qf-ratios in each stage. If, for example, two hydrocyclone stages are used, the ratio Qa/Qf in the first stage can be kept within the interval 0.10-0.40, whereas the ratio in the second stage can be kept on a lower level, such as 0.05-0.25.
As for the dimensions of the hydrocyclones for separation of fibers with low bonding ability, when no blocking device is used, the preferred ratios between the length (Lc) and the greatest cone diameter (Dc) is kept within the interval 5.2-6.5, the ratio between the base outflow diameter (Db) and the greatest cone diameter (Dc) is kept within the interval 0.10-0.20, the ratio between the apex outflow diameter (Da) and the greatest cone diameter (Dc) is kept within the interval 0.18-0.30, and the ratio between the base outflow diameter Db and the apex outflow diameter (Da greater than ) is kept less than 1.
When a blocking device is used, the dimensions of the hydrocyclones are the same as described above with the exception of the ratio between the base outflow diameter(Db) and the greatest cone diameter (Dc) which is kept within the interval 0.10-0.26.
The ratio of the diameter (Dd) of the blocking device at the end (E) to the diameter (Db) of the base outflow opening is preferably kept within the interval of from 0.1 to 0.9 when the blocking device is arranged within a central outlet tube (T) at the base end of the hydrocyclone and extending axially from the base outflow opening into the hydrocyclone chamber. Such extension can preferably be from 0 to 5 times the diameter (Db) of the base outflow opening. It is also possible to arrange the blocking device within the central tube (T) at the base end of the hydrocyclone, extending axially with its end (E) within this tube at a distance of from 0 to 5 times the diameter (Db) of the base outflow opening in the flow direction from the base outflow opening. In the latter case it is also possible to make the central tube (T) widening in the flow direction, and the diameter (Dd) of the end (E) of the blocking device greater than the diameter (Db) of the base outflow opening.
According to the invention it is also suitable to treat rejects from the hydrocyclones for separation of fibers with low bonding ability in one or more hydrocyclones designed for separation of sand, bark and heavy particles, and this treatment can be carried out in one or more hydrocyclone stages. In this case it is preferred that the ratio Qa/Qf is kept within the interval 0.05-0.10, and the ratio between the base outflow diameter (Db) and the apex outflow diameter (Da) is kept greater than 1.